Of all the strands of thought, tradition, and belief that make up the Islamic universe, Sufism in its doctrinal aspect stands out as the most intact, the most purely Islamic: the central strand. Opponents of Sufism often charge it with having originated outside Islam, but a close study of the various schools of philosophy and theology, and a comparison with "primordial" Islam as revealed in the Koran and hadith (authentic sayings of the Prophet Muhammad), will vindicate the Sufis' claim of centrality, of strict adherence to the original purity of the Revelation.

In the context of the history of thought, in fact, Sufism - always insisting on a return to the sources of the Tradition - can be seen to have functioned at times as a positive and healthy reaction to the overly rational activity of the philosophers and theologians. For the Sufis, the road to spiritual knowledge - to Certainty - could never be confined to the process of rational or purely intellectual activity, without sapiential knowledge (zawq, "taste") and the direct, immediate experience of the Heart. Truth, they believed, can be sought and found only with one's entire being; nor were they satisfied merely to know this Truth. They insisted on a total identification with it: a "passing away" of the knower in the Known, of subject in the Object of knowledge. Thus, when the fourth/tenth century Sufi Hallaj proclaimed "I am the Truth" (and was martyred for it by the exoteric authorities), he was not violating the "First Pillar" of Islam, the belief in Unity (tawhid), but simply stating the truth from the mouth of the Truth. So the Sufis believe.